Clock synchronization

Clock synchronization is a topic in computer science and engineering that aims to coordinate otherwise independent clocks. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at slightly different rates. There are several problems that occur as a result of clock rate differences and several solutions, some being more appropriate than others in certain contexts.[1]

As a result of the difficulties managing time at smaller scale, there are problems associated with clock skew that take on more complexity in a distributed system in which several computers will need to realize the same global time.

For instance, in Unix systems the make command is used to compile new or modified code without the need to recompile unchanged code. The make command uses the clock of the machine it runs on to determine which source files need to be recompiled. If the sources reside on a separate file server and the two machines have unsynchronized clocks, the make program might not produce the correct results.[3]

In a distributed system the problem takes on more complexity because a global time is not easily known. The most used clock synchronization solution on the Internet is the Network Time Protocol (NTP) which is a layered client-server architecture based on UDP message passing. Lamport timestamps and vector clocks are concepts of the logical clocks in distributed systems.

Cristian's algorithm relies on the existence of a time server.[4] The time server maintains its clock by using a radio clock or other accurate time source, then all other computers in the system stay synchronized with it. A time client will maintain its clock by making a procedure call to the time server. Variations of this algorithm make more precise time calculations by factoring in network radio propagation time.

The Berkeley algorithm is suitable for systems where a radio clock is not present, this system has no way of making sure of the actual time other than by maintaining a global average time as the global time. A time server will periodically fetch the time from all the time clients, average the results, and then report back to the clients the adjustment that needs be made to their local clocks to achieve the average. This algorithm highlights the fact that internal clocks may vary not only in the time they contain but also in the clock rate.

Often, any client whose clock differs by a value outside of a given tolerance is disregarded when averaging the results. This prevents the overall system time from being drastically skewed due to one erroneous clock.

Network Time Protocol (NTP) is a highly robust protocol, widely deployed throughout the Internet. Well tested over the years, it is generally regarded as the state of the art in distributed time synchronization protocols for unreliable networks. It can reduce synchronization offsets to times of the order of a few milliseconds over the public Internet, and to sub-millisecond levels over local area networks.

A simplified version of the NTP protocol, SNTP, can also be used as a pure single-shot stateless master-slave synchronization protocol, but lacks the sophisticated features of NTP, and thus has much lower performance and reliability levels.

CS-MNS is suitable for distributed and mobile applications. It has been shown to be scalable over mesh networks that include indirectly linked non-adjacent nodes, and compatible to IEEE 802.11 and similar standards. It can be accurate to the order of few microseconds, but requires direct physical wireless connectivity with negligible link delay (less than 1 microsecond) on links between adjacent nodes, limiting the distance between neighboring nodes to a few hundred meters.[5]

The Reference Broadcast Time Synchronization (RBS) algorithm is often used in wireless networks and sensor networks. In this scheme, an initiator broadcasts a reference message to urge the receivers to adjust their clocks.

The Reference Broadcast Infrastructure Synchronization (RBIS)[6] protocol is a master/slave synchronization protocol based on the receiver/receiver synchronization paradigm, as RBS. It is specifically tailored to be used in IEEE 802.11 Wi-Fi networks configured in infrastructure mode (i.e., coordinated by an access point). The protocol does not require any modification to the access point.